Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole is referred to as “logging.”
Logging frequently is done during the drilling process, eliminating the necessity of removing or “tripping” the drilling assembly to insert a wireline logging tool to collect the data. Data collection during drilling also allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
When oil wells or other boreholes are being drilled, it is frequently necessary or desirable to determine the direction and inclination of the drill bit and downhole motor so that the assembly can be steered in the correct direction. Additionally, information may be required concerning the nature of the strata being drilled, such as the formation's resistivity, porosity, density and its measure of gamma radiation. It is also frequently desirable to know other downhole parameters, such as the temperature and the pressure at the base of the borehole, for example. Once this data is gathered at the bottom of the borehole, it is typically transmitted to the surface for use and analysis by the driller.
Sensors or transducers typically are located at the lower end of the drillstring in LWD systems. While drilling is in progress these sensors continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. Typically, the downhole sensors employed in LWD applications are positioned in a cylindrical drill collar that is positioned close to the drill bit. The LWD system then employs a system of telemetry in which the data acquired by the sensors is transmitted to a receiver located on the surface. There are a number of telemetry systems in the prior art that seek to transmit information regarding downhole parameters up to the surface without requiring the use of a wireline tool. These include the mud pulse telemetry system and the through-drillstring telemetry system.
The mud pulse telemetry system creates acoustic pressure signals in the drilling fluid that is circulated under pressure through the drillstring during drilling operations. The information that is acquired by the downhole sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. The information is received and decoded by a pressure transducer and computer at the surface.
The through-drillstring telemetry system transmits data using vibrations in the tubing wall of the drillstring. The vibrations are generated by an acoustic transmitter (e.g., piezoelectric washers) mounted on the tubing wall of the drillstring and are transmitted upstream to an acoustic receiver (e.g., an accelerometer), also mounted on the drillstring tubing wall. Several transmitter/receiver pairs may be positioned along the length of the drillstring acting as repeaters. The information is received and decoded by an acoustic receiver and computer at the surface.
Because these systems are acoustic in nature, their signals are susceptible to distortion by ambient noise and vibration. In an environment such as a drilling rig there can be a large variety of acoustical noise and vibration sources. The presence of noise and vibrations in the drillstring due to activities surrounding the drilling process severely hinders the detection of acoustic telemetry signals.